Nearly half a million locomotives and freight cars form trains that transport a significant proportion of the United States' freight every year. These trains crisscross the country and can: measure over a mile in length; have a mass well over 20,000 tons; and move at speeds of fifty miles per hour or more. To slow and stop such trains requires brakes on each railcar. These brakes, similar to brakes on any other vehicle, will wear out and can malfunction in various ways. The failure of a brake can have many consequences, ranging from a damaged wheel to a derailment or a collision, which can result in many deaths.
FIG. 1 shows an illustrative rail car shoe brake according to the prior art. A railroad wheel 2 is generally a solid steel object with a number of component features, including a tread 3 on which the wheel 2, and therefore the rail vehicle, rests on a rail. A brake assembly 4 includes a brake pad 5 and a brake shoe 7, which are held together by a wave spring (key). The spring permits removal of the brake pad 5 whenever the brake pad 5 becomes too worn. The brake assembly 4 is attached to some device or mechanism, which can move the brake pad 5 into contact with the tread 3 of the wheel 2. This contact causes a dissipation of energy from the wheel 2 and thereby causes the train to slow.
However, the contact and friction also causes the brake pad 5 to wear. Eventually, the brake pad 5 will need to be replaced. However, the railroad industry does not have a well-established solution for routinely inspecting the condition of railcar brakes. To date, brake inspections are generally performed during incoming inspections, require the rail vehicle to be at a full stop, and merely involve an inspector examining the brakes “by eye.” If the inspector has any doubt as to the condition of the brake pad 5, a metal ruler can be used to determine if the thickness remaining on the brake pad 5 is sufficient for its continued use.